It is conventionally known as a method of cleaning such workpiece as electronic elements, for example, that a workpiece is immersed in a cleaning solution containing an inert solvent as a main component and stored in an ultrasonic cleaning bath for cleaning and removing such foreign substances as greases and chips adhered to surfaces of the workpiece by cavitation of the cleaning solution by means of an ultrasonic oscillator.
However, in such ultrasonic cleaning of a workpiece by using a cleaning solution containing an inert solvent as a main component as described above, however, it is a problem that a high cleaning performance cannot be obtained, because the solution is lower in cleaning performance of separating and removing greases adhered to the workpiece in comparison with a cleaning solution containing hydrocarbon solvent as a main component.
Although the above problem can be solved by ultrasonically cleaning a workpiece with a cleaning solution containing a hydrocarbon solvent as a main component, after the cleaning operation, when no more element for deaeration is present, such as when the ultrasonic is discontinued, because the air is dissolved in the cleaning solution, and propagation of the ultrasonic oscillation is inferior, a cleaning effect by the hydrocarbon solvent cannot be expected. In addition, as hydrocarbon solvents is higher in surface tension and lower in permeability than inert solvents, it is difficult to clean and remove foreign substances adhered in spacings and blind holes of a workpiece, and an isolating means for preventing it from being in contact with the air that may cause inflammation must be further provided, thus, requiring a careful handling.
Moreover, in the case of using both hydrocarbon and inert solvents, because a cleaning bath for storing the hydrocarbon solvent and another for the inert solvent are independently employed, it is another problem that an entire apparatus comes to be complicated in structure and bulky.